Exploratory and seasonal movements of adult common shelducks in the eastern Wadden Sea

Habitat conditions surrounding individual birds are key components determining their fitness, notably by influencing effective foraging, escape from predators, and adaptation to changing environmental conditions. Birds living in dynamic habitats are particularly likely to have a high need to explore; however, spatial and temporal patterns of exploratory movements are poorly understood. We investigated the exploratory movements of adult common shelducks (hereafter Shelducks, Tadorna tadorna) in the Wadden Sea, which provides a dynamic habitat for birds. We captured nine shelducks in adjacent breeding areas and attached GPS satellite telemetry devices for 5–18 months, mostly covering the post-breeding period. We analyzed bird-movement patterns based on mean daily locations and behavioral change point analysis. All individuals explored on a median of 22% of tracking days (range = 6–39%). Individuals usually explored new areas for a short time before switching to stay in these areas for longer periods. Exploratory movements were particularly frequent before the start of the flightless period during molting. During this time, shelducks explored the traditional molting area in the Elbe estuary intensively. However, birds did not explore places within the area subsequently used for molting for longer or more often than other places. The molting area was used for a median of 59 days, i.e., about twice as long as required for the flightless period, suggesting that this area was explored not only to select a suitable molting place, but also for other purposes. Only two shelducks performed longer-distance exploration movements, i.e., > 45 km from their sedentary areas. We therefore concluded that exploratory movements by adult shelducks may enable the birds to adapt to small-scale changes in habitat conditions.